The rear backup sonar won't turn on in my 05 Armada. I just bought the car a few weeks ago, and it hasn't ever worked. I read the threads here and found very helpful information. I checked the fuses (all good) and ran the diagnostic, which tells me there is a problem with the buzzer (code 21). The buzzer didn't sound during the diagnostic, and the forums make it sound as though that's a common problem. So, I paid the $45 for a new buzzer and made the swap. It didn't change anything. The off switch is still illuminated, the diagnostic still says there is a buzzer problem, and the buzzer still doesn't sound. Anyone have any ideas about what to check next? Do I look at the sonar control unit next? Anyone have the service manual pages for replacing the control unit?